1. Field of the Invention
This invention relates to a photo-stabilized cyanine dye and an optical recording medium having a dye film as a recording layer. More particularly, it relates to a photo-stabilized cyanine dye which allows a dye coating step of a disc manufacturing process to use a solvent having a high evaporation rate, and a write-once type optical recording medium, especially optical disc, using the same.
2. Description of the Background
In recent years, various optical recording discs of the write-once, rewritable and other types are used in plenty as high capacity information carrying media. Among the optical recording discs, there are known those having a dye film composed mainly of a dye as the recording layer. From a structural aspect, optical recording discs proposed thus far include discs of the air-sandwich structure having an air space on a dye film and discs (CD-R) having a reflective layer of gold or the like disposed in close contact with a recording layer made of a dye film for providing a higher reflectance which can be read in accordance with the compact disc (CD) standard. (See Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and Proceedings SPIE--The International Society for Optical Engineering, Vol. 1078, pages 80-87, "Optical Data Storage Topical Meeting", 17-19, January 1989, Los Angels.) The demand for CD-R is dramatically increasing in these years. Since the quantity of CD-R consumed is increasing as rapidly as the supply is not equal to the demand, the productivity as to how many products can be supplied within a short time becomes an important task.
The demand for higher density recording is also increasing, which requires to reduce the wavelength of recording lasers. The digital video discs (DVD) complying with a laser having a wavelength of about 635 nm, on which efforts have been made for standardization as the next generation recording medium, are now on the verge of commercial products. The development of DVD-R using dyes for single recording use is also in progress. The dyes used therein must also be compatible with shorter wavelengths.
In the above-described discs, recording layers are generally formed by applying a coating dye solution.
When it is desired that such recording layers comply with CD-R and DVD-R, the use of cyanine dyes as the dyes for recording is preferred because they have optical advantages including easy alternation of wavelength and a greater index of refraction. On the other hand, the cyanine dyes have the drawback of lacking stability against light. Phthalocyanine dyes are well stable against light although it is difficult to use them in DVD-R because they cannot be tailored for shorter wavelength.
For the stabilization of cyanine dyes, the inventors found that salt forming dyes (ionic bond compound) between a benzenedithiol metal complex anion which is a singlet oxygen quencher and a cyanine dye cation are significantly stable against light, improved in light fastness and reduced in deterioration by reading (Japanese Patent No. 1551668 etc.), and succeeded in practical use thereof.
However, cyclohexanone which is a suitable solvent for dissolving the salt forming dye cannot be used because it attacks polycarbonate substrates. Accordingly, solvents having a fairly low drying rate such as diacetone alcohol must be used. Owing to the slow drying rate of solvents, a longer coating time is required per optical disc, becoming an obstruction against productivity improvement. Although cyanine dyes themselves are soluble, for example, in TFP (2,2,3,3-tetrafluoropropanol) at relatively high concentrations, most salt forming dyes substantially lose solubility. Even when dissolvable, they often fail to reach concentrations sufficient to provide a necessary film thickness.
Furthermore, the ionic bond compounds of cyanine dyes using conventional quencher anions have the problem that their effect on trimethine and monomethine cyanine dyes for the short wavelength is inferior to their effect on conventional heptamethine cyanine and pentamethine cyanine dyes for the long wavelength.